The progress of the past year is summarized under arbitrary groupings: Immunology: studies of thymus-leukemia antigen expression and modulation. Developmental Biology: the use of lactoferrin as a marker of granulocyte production, the histogenetic properties of glial tumor cells, the relationship between erythroid and granulocyte differentiation and the role of purified tissue-specific cell-aggregation factors in cellular interactions. Cell and RNA Replication: studies of repetitive DNA sequences of mitochondrial DNA and of thermal shock effects, studies of DNA repair by human cells and the use of yeast mutants to investigate recombination. Transcription: studies of the molecular mechanism of action of anti-tumor compounds and the nature of the protein RNA complexes that are formed in hepatoma cells and contain heterogeneous RNA and polyadenylated RNA. Translation: studies of the mode of action of insulin and the biosynthesis of preproinsulin by tumor cells, the regulation of tRNA thiolation by a tumor derived inhibitor and the structure and function of ribosomal proteins.